In a QAM symbol transmission system of the type disclosed, a transmitted data symbol is represented by both "I" and "Q" quadrature components which modulate respective quadrature phased carriers at a given frequency. Each symbol may comprise several bits, and the number of bits/symbol dictates the type of QAM system, i.e., 16-QAM, 32-QAM, etc. Each symbol is mapped (assigned) to a prescribed coordinate in a four-quadrant grid-like constellation using a look-up table (e.g., a ROM). A prescribed number of symbols occupy assigned areas in each quadrant. In a 32-QAM system, each quadrant of the constellation contains eight symbols at prescribed coordinates with respect to quadrature I and Q axes. Certain symbol bits designate the constellation quadrant in which a symbol is located, and certain bits designate the particular coordinate in that quadrant assigned to the symbol. QAM systems of this general type are well known.
In the system disclosed in White and in Raychaudhuri et al., a television signal representing high definition image information is transmitted using two QAM carriers frequency multiplexed in a standard 6 MHz television transmission baseband. One of the carriers conveys high priority information, while the other carrier conveys (relatively lower) standard priority information. The high priority (HP) information is the information needed to create a viewable image, although less than a perfect image, and is conveyed with significantly more power than the standard priority (SP) information, which is the remaining information. The high priority information exhibits a narrow bandwidth compared to the standard priority information, and is therefore much less prone to corruption by the transmission channel. The HP carrier is located in that portion of the frequency spectrum of a television transmission channel, e.g., an NTSC channel, which is normally occupied by the vestigial sideband of a standard NTSC television signal. This portion of the signal is normally significantly attenuated by the Nyquist filters of standard receivers, so that HDTV signals with this transmission format will not introduce co-channel interference.
One approach to decoding such a dual QAM signal, as disclosed by White, employs two parallel signal paths for processing high priority and standard priority signal components, respectively. Each path contains an adaptive equalizer. In addition, each path contains a derotator/demodulator responsive to an error signal for carrier recovery (correction) purposes. The complex (I,Q) adaptive equalizers in each of the parallel signal paths of White represent large, complex and expensive hardware components.
A system for reducing the amount of such parallel hardware to produce a less expensive system is described in a copending Lauren A. Christopher U.S. Pat. No. 5,263,018, "Apparatus for Time Division Multiplexed Processing of Plural QAM Signals", filed Jul. 29, 1992. To this end the Christopher system employs time division multiplexed processing of high priority and standard priority components of a received composite signal. While advantageously reducing the hardware requirements compared to the White system, the system described by Christopher introduces the added complication of time division multiplexing. The signal processing system described herein offers a further refinement of a prioritized QAM decoder by advantageously avoiding both the hardware requirements of White and the time division multiplex complexity exhibited by Christopher, without compromising the quality of a decoded QAM signal.